The low-resolution tachometers currently available for use on Industry Standard Frame Size AC and DC motors generally consist of a magnet biased sensor magnetically coupled to a metal gear-toothed wheel (the wheel being fastened to rotate with the DC motor output shaft). Varying pulse counts of 60, 120 and 240 pulses per revolution ("PPR") are typically obtained by providing an appropriate number of teeth on the gear-toothed wheel. Such arrangements are useful in certain low-resolution applications, but have several disadvantages in other applications.
One major disadvantage of such standard low resolution tachometers is that different wheels must be provided for different pulse counts. This means that the manufacturer must make (and inventory) several different types of toothed wheels having different numbers of teeth; and that original equipment manufacturers must keep a stock of the various different wheel types in inventory. Thus, the use of different types of wheels for different resolutions and applications increases design, manufacturing and inventory costs.
In addition, the resolution of such prior art low resolution arrangements is currently limited to about 240 pulses per revolution ("PPR"). This is because the upper limit on pulse count is a function of how many teeth can accurately be placed on the wheel; and on the ability of the sensor to differentiate between the teeth. Mechanical durability and machining tolerances limit the number of teeth that can be placed on the wheel. Moreover, speed variations, the size of the magnetic sensor, and other factors require a minimum spacing between teeth to ensure that each tooth is detected.
A further disadvantage of prior art arrangements is that an auxiliary sensor is required to provide directional information. This auxiliary sensor must be mechanically positioned so that a "quadrature" signal is obtained. Such a quadrature signal typically provides two pulse signals that are 90 degrees out of phase with respect to one another, and thus permit digital circuitry to accurately derive higher resolution outputs therefrom. Quadrature signals are commonly provided by conventional shaft encoders. Moreover, there are known methods for scaling the timing of such quadrature signals while maintaining the scaled output signals in quadrature. For example, it is known to pass quadrature signals through an Exclusive OR logic function to provide a frequency doubling effect. As another example, DynaPar manufactures a "divide by four" electrical module that accepts quadrature input signals in quadrature and scales those input signals by a "divide by 4" operation to provided scaled quadrature output signals. One technique used in the past for scaling of quadrature output signals is to synthesize quadrature output waveforms using an EEPROM lookup table.
It is generally known to use magneto-resistive sensor assemblies to ascertain shaft rotation parameters. The following is a non-exhaustive but somewhat representative list of prior patents relating to magneto-resistive sensor assemblies:
U.S. Pat. No. 4,656,377 to Akiyama et al; PA1 U.S. Pat. No. 4,866,382 to Carmen; PA1 U.S. Pat. No. 4,853,631 to Carmen; PA1 U.S. Pat. No. 4,851,771 to Ikeda et al; PA1 U.S. Pat. No. 4,274,053 to Ito et al; PA1 U.S. Pat. No. 4,319,188 to Ito et al; PA1 U.S. Pat. No. 5,019,776 to Kawamata et al; PA1 U.S. Pat. No. 4,786,870 to Kawamata et al; PA1 U.S. Pat. No. 4,853,632 to Nagano et al; PA1 U.S. Pat. No. 4,639,807 to Sekizawa et al; PA1 U.S. Pat. No. 4,766,376 to Takahashi et PA1 U.S. Pat. No. 4,818,939 to Takahashi et al; PA1 U.S. Pat. No. 3,764,888 to Anderson; PA1 U.S. Pat. No. 4,988,220 to Christiansen et al; PA1 U.S. Pat. No. 4,890,059 to Guentner; PA1 U.S. Pat. No. 3,997,782 to Willits. PA1 U.S. Pat. No. 4,992,733 to Griebeler; PA1 U.S. Pat. No. 5,041,784 to Griebeler; PA1 U.S. Pat. No. 4,857,841 to Hastings et al; PA1 U.S. Pat. No. 4,914,389 to Juds; PA1 U.S. Pat. No. 4,922,197 to Juds et al; PA1 U.S. Pat. No. 4,816,948 to Kamo et al; PA1 U.S. Pat. No. 4,874,053 to Kimura et al; PA1 U.S. Pat. No. 4,914,387 to Santos; PA1 U.S. Pat. No. 4,668,913 to Vinal. PA1 U.S. Pat. No. 4,975,675 to Becker; PA1 U.S. Pat. No. 4,712,064 to Eckhardt et PA1 U.S. Pat. No. 4,418,372 to Hayashida et al; PA1 U.S. Pat. No. 4,800,457 to Kryder et PA1 U.S. Pat. No. 4,506,217 to Rothley et al; PA1 U.S. Pat. No. 4,686,472 to Van Ooijen et al.
Carmen '382 teaches a magneto-resistive element array which is selectively connected to accommodate a number of different diameter encoder wheels each containing a different number of pieces of magnetic information (e.g., ranging from 100 to 512 poles). Output signals are provided in quadrature to increase the resolution of the system (e.g., to provide a frequency equal to four times the pulse count, e.g., 400 to 2048 PPR). Comparators are used to generate output pulses. The system senses the number of rotations (i.e., using an index mark on a further circumferential track) in addition to incremental movement of the encoding wheel. See also Carmen '631, which teaches bridging magneto-resistive sensor outputs to sense information recorded at different frequencies.
Ito et al '053 and Ito et al '188 relate to multitrack magnetic rotary encoders for use in determining the rotating conditions of a rotatable shaft. Encoders are described for mounting on the face plate of an electric motor (see FIG. 17 of Ito '053, and FIG. 19 of the Ito '188 patent). Different sensor array configurations are used to provide different phased signals. An integrated circuit rotary position detector 14 is disclosed in the '188 patent as disposed within a face plate sensor housing. Both analog and BCD output signals are provided.
Nagano et al teaches a magneto-resistive sensor for determining position of a motor shaft. A gear-shaped disk of magnetic material is fixed to a hub, and the hub is mounted to the shaft of a motor (see FIG. 1). The hub and disk are mounted within an enclosure on the motor face plate. A pair of magneto-resistive elements 7 and associated signal processing integrated circuit are mounted on an insulating substrate within the enclosure. The signal processing circuitry includes a waveform shaping circuit having a comparator.
Ikeda et al teaches various embodiments of magneto-resistive encoders and associated sensors for determining speed and position. Their disclosure is directed to the orientation between a magneto-resistive "stripe" relative to magnetic patterns disposed on an encoding drum. Multiple tracks and multiple sensors provide speed and displacement information.
Akiyama et al teaches using dual magneto-resistive sensors for determining direction.
Kawamata et al '776 and Kawamata et al '870 relate to multitrack magneto-resistive position detectors. These patents disclose different magneto-resistive element configurations and connections to provide proper combinations of the multi-track signals.
Sekizawa et al teaches a magneto-resistive speed detector including an array of magneto-resistive elements connected in a bridge circuit.
Takahashi et al '939 discloses a rotating drum type magneto-resistive speed sensor arrangement providing multiple sensing elements that provide associated differently phased output signals. Such output signals are combined to provide position information. Takahashi et al '376 discloses a multitrack drum coupled to a motor output shaft to provide an absolute position detector. Voltage comparators and combinatorial logic elements are used to provide position signals based on multiphase input signals from the magneto-resistive sensor elements.
The following additional references disclose various features of shaft sensors (not in the context of magneto-resistive type sensors, however):
Christiansen et al teaches a wheel speed sensor assembly that can be partially disassembled without exposing the wheel bearing.
Guentner teaches a non-contacting digital tachometer including wire pulse sensors separated from a magnetized drum surface by an air gap. Two adjusting holes are aligned with the air gap to permit the legs of an adjusting yoke to be inserted and establish the air gap between the sensors and the wheel.
Willits teaches a photoelectric rotary pulse transducer including a cylindrical housing.
Anderson teaches a direct current teachometer system which provides a DC output having a magnitude which varies with tachometer speed. Reference pulses are generated exhibiting a frequency proportional to tachometer speed. A control circuit varies the magnitude of the DC output signal in dependence on the frequency of the reference pulses.
The following additional references relate generally to magneto-resistive sensing techniques:
Griebeler -733 and Griebeler '784 teach transducers for use in measuring the parameters of a linearly moving ram within a die casting machine.
Hastings teaches a magneto-resistive proximity sensor.
Juds '389 teaches a multi-turn shaft position sensor. Juds et al '197 teaches a high resolution proximity detector.
Kamo et al teaches a way to magnetize a magneto-resistive film.
Kimura et al teaches magneto-resistive torque (shaft torsion measuring) sensors with various signal processing arrangements (including some software based signal processing algorithms).
Santos teaches a magneto-resistive speed sensor that adapts to changes due to aging, ambient magnetic field, etc.
Vinal teaches a constant flux magneto-resistive magnetic reluctance sensor for reading magnetic ink characters.
The following references relate to magneto-resistive probe assemblies per se:
Hence, much work has been done in the past in the area of tachometers in general and magneto-resistive tachometers in particular. However, further improvements are possible.
The present invention provides a new type of tachometer that overcomes the problems discussed above and other problems as well.
Briefly, the preferred embodiment "Magnetic Incremental Encoder" magneto-resistive sensor arrangement provided by the present invention utilizes a non-magnetically biased magneto-resistive sensor element; a magnetized drum; and associated integrated circuit based electronics. More specifically, the sensor arrangement includes an enclosure, a drum, a magneto-resistive probe, a hub, and an electronics module. The generally cylindrical enclosure is bolted onto the end plate of a motor casing or housing (e.g., a standard NEMA 4.5 inch and 8.5 inch C-Face end plate of an AC or DC electric motor). The enclosure provides a circular opening for the motor shaft to protrude through. In one exemplary embodiment, the hub is fastened to the end face of the motor shaft, and a magneto-resistive drum is fastened to the hub. The drum includes two magnetized tracks: an incremental magnetic ("INC") track and an index pulse ("Z") track. Such magnetized tracks are formed in a conventional manner by magnetizing the periphery of the drum. The "Z" track encodes one pattern (pulse) per revolution, while the "INC" track encodes (in the preferred embodiment) 480 patterns (pulses) per revolution.
The enclosure is formed with at least one, and preferably a pair of planar saddle surfaces, each having a rectangular opening therethrough. In one exemplary embodiment, a generally rectangular sensor housing or module is detachably fastened to the saddle surface and protrudes radially through the rectangular opening into the interior of the enclosure. The sensor housing supports a two track magneto-resistive probe at a free end thereof for sensing the "INC" track and the "Z" track, respectively. The preferred embodiment probe comprises a conventional magneto-resistive sensing element(s) and a miniature PC board providing signal processing circuitry.
The mounting arrangement between the enclosure and motor end plate, the enclosure and the sensor module, and the motor shaft and hub/drum provides a self-gapping probe-to-drum spacing of approximately 0.020 plus or minus 0.009 inches, and provides for self-alignment (axially and radially) between the probe and the drum. Thus, one significant innovation of the preferred embodiment design is that the magnetic drum and mounting hub, enclosure, and sensor module can be assembled onto a motor housing and put into operation without extensive alignment procedures. In other words, after the magnetic drum and hub have been fitted to the end face of the motor shaft and bolted in place, the enclosure is then bolted into place on the motor housing end plate. Finally, the sensor module is slipped into the radial opening in the enclosure and bolted into place on the saddle surface. The wires are connected and the tachometer is ready to operate, with no special adjustment of the location of the probe or sensor module required. With this configuration, the probe is removable and replaceable with another probe without any realignment of the assembly. This feature is advantageous for both assembly and service repair.
Such self-alignment and self-gapping is possible because electric motors are generally produced in accordance with NEMA standards which set the dimensions and tolerances of standard motor facings and shaft sizes. The preferred embodiment enclosure is machined to fit onto NEMA C type motor faces to a tight tolerance. The sensor module saddle mounting surface is also machined on the side of the enclosure to a tight tolerance with respect to the holding rim or lip of the enclosure which interfaces with the motor end plate. By holding these tight tolerances, as well as tight tolerances on the depth of the sensor from the mating surface of the probe to the surface of the sensor, the sensor can be located at a very precise distance from the center-line of the motor shaft.
In a like manner, the hub which is used to hold the magnetic drum onto the motor shaft is also machined to high tolerances. In one exemplary embodiment, the hub has a recess which fits over the end of the motor shaft. The recess is concentric to the center-line or axis of the shaft and to the outer diameter of the hub. Onto this the drum is centered with respect to the outer diameter of the hub and held in place by screws or other suitable means (e.g., structural adhesive). The outer diameter of the drum is also machined to a tight tolerance. The depth of the hub recess determines the longitudinal location of the drum with respect to the sensor, and therefore, this depth is also tightly controlled.
In this configuration, the drum and sensor can be precisely located with respect to each other, across a radial gap. The tolerance of the gap between the sensor and drum edge is a function of the dimensional tolerances of the individual parts. According to the NEMA standards for one application, for example, the critical dimensions of the motor facing eccentricity is four thousands of an inch TIR (0.004"). The TIR of the shaft is three thousands of an inch (0.003"). The machining tolerances of the enclosure, probe and hub can be held to a few thousands of an inch or less. By using the tight machining tolerances as described above, the tolerance of the radial gap can be kept to plus-or-minus ten thousands of an inch (+/-0.010") or better.
Other physical arrangements in different motor applications are within the scope of this invention. For example, for those motors which have relatively longer output shafts (which may be connected at their free end to a driven element for example), the enclosure is provided with an aperture to allow the motor shaft to extend therethrough. In addition, the above described hub is replaced by a sleeve type hub which may be located axially along the shaft and set at a precise distance from the motor end plate. Otherwise, the sleeve type hub functions much in the same manner as the first described hub in that a magnetic drum is fixed thereto for precise orientation vis-a-vis the magnetic probe.
In still another exemplary embodiment, a hub is provided for a motor output shaft end as in the first described embodiment, but with the further provision for some small axial adjustment relative to the end surface of the shaft using fine thread set screws. This is to accommodate any axial run-out of the shaft relative to the motor beyond the stated tolerance.
In the preferred embodiment, the sensor module and probe includes associated electronics which provides a square wave pulse train for use in position and velocity feedback and control applications. The sensor arrangement in accordance with one exemplary embodiment can produce pulse counts of 60, 64, 75, 120, 128, 150, 240, 256, 300, 480, 512, 600, 960, 1024 and 1200 pulses per revolution (the pulse count being set via jumpers coupled to the electronics module). The matched magneto-resistive sensor and drum provide a pulse count of a nominal frequency (e.g., 480, 512 or 600 PPR) dependent on the magnetic pattern provided on the magnetized drum. In order to generate quadrature maintained pulse counts, a low power programmable logic device is used to divide-by-n the "raw" output frequency to provide derived pulse counts in quadrature maintained form.
Briefly, a programmable asynchronous state machine is used to selectively divide by 2, 4 or 8 while maintaining quadrature. Base logic divides by a certain factor (e.g., by 2 to yield 240). Further divisions are used to obtain the desired pulse rate. Jumpers specify the factor of n in the presently preferred embodiment (programming can be accomplished by routing the appropriate set of signals to the output drivers via jumpers at the time of order). The preferred embodiment system provides a differential output, with the following output phases: A phase, B phase 90.degree. phase gap relative to phase A (when A phase leads B phase, the monitored rotation is clockwise); and Z phase. Such outputs are provided at a rugged, environmentally sealed connector on the top of the sensor holding in the preferred embodiment.
The preferred embodiment sensor provided by the present invention thus provides a two-channel incremental quadrature output as well as a once-per-revolution index or marker pulse. A common drum (and common electronic circuitry) is used for multiple pulse counts of a single divisor. In other words, one drum encoding 480 pulses per revolution as a "raw" pulse count, can be used to obtain incremental pulse output rates of 480, 240, 120 or 60 pulses per revolution; a drum encoding 512 pulses can be used to obtain incremental pulse output rates of 512, 256, 128 and 64 pulses per revolution; and a drum encoding 600 pulses per revolution can be used to obtain incremental pulse ratio of 600, 300, 150 and 75 pulses per revolution.
It will be appreciated that different drums are required for encoding 480, 512 and 600 pulses because, in order to maintain the same pitch pattern, the diameters of the respective drums must necessarily change. In addition, in order to maintain the same radial gap for each drum/sensor combination, it is necessary to provide a precisely machined spacer for insertion between the common sensor module and the saddle surface of the enclosure. Thus, if the enclosure, sensor module and drum are calibrated in the basis of a 480 pulse drum, no spacer is required. For the 512 and 600 pulse drums, however, a spacer would be required for each, as determined by the respective diameter increases over the 480 pulse drum. In this way, a common sensor module may be used with any of the above described drums.
Even with the few additional parts described above, the invention nevertheless provides a considerable reduction in inventory otherwise required to provide the wide range of pulse count options possible with this invention (e.g., the manufacturer needs to estimate only the total number of sensors, drums and spacers that will have to be delivered, and does not have to worry about anticipating what particular pulse counts customers will require). The electronics/sensor package is easily separable from the mounting enclosure so that field adjustment and/or replacement can be performed without removal of the sensor enclosure. No electrical adjustments or alignment are required by the installer, and the sensor assembly is self-gapping and self-aligning. Programmable divide-by-n and/or multiply-by-n circuitry within the magneto-resistive tachometer housing permit a common drum and sensor to be used to provide different pulse counts.